Method and apparatus for assembling frames

ABSTRACT

In a method for assembling frames of a multiframe first the value of an identifier (c) for inclusion within each of said frames is calculated before these are transmitted at the rate of a transmitter clock (CLKTX). The identifier is calculated such that the amount of distinct values (m) said identifier can have is less than the amount of frames (f) within said multiframe. This identifier is extracted from said frames in a receiver, and is used for determining a position in a temporary buffer (CM) of which the amount of memory locations (x) is smaller than said amount of frames (f) within said multiframe. The frames are then read from positions in said temporary buffer for storage into subsequent locations into a multiframe buffer memory, irrespective of the value of said identifier (c). The receiver clock is thereby synchronised (CLKRX) with the transmitter clock (CLKTX). A transmitter, frame receiver device, receiver and communications network wherein this method is performed are described as well.

[0001] The present invention relates to a method for assembling frames of a multiframe, as is described in the preamble of claim 1, and to a frame receiver device for performing such a method as is described in the preamble of claim 8.

[0002] It is well known that in many ATM-TDM systems, such as for instance the APON system, which is the abbreviation of asynchronous passive optical networks, ATM cells transmitted at a transmitter, for instance an optical line termination unit, hereafter abbreviated as OLT, have to be recombined in the correct sequence at the receiver. This is especially true in case Channel Associated Signalling, is used. In this case a multiframe is to be composed of 16 previously sent TDM frames, to be completed with specific Channel Associated Signalling information for further use by a downstream user. In case these frames can arrive asynchronously, and not in the sequence they were previously sent by the transmitter, for instance due to cell delay variation problems, the only solution for properly assembling them in the multiframe was to first temporarily buffer them in a temporary buffer memory, after which step, if all 16 were received, they could be read out from this memory for assembly into the multiframe buffer memory. Such a straightforward method thereby not only requires a large temporary buffer memory, but also introduces an additional delay because of this temporary storage action. Furthermore this solution implies that within each transmitted frame, an identifier having a number of 1 to 16 is to be included, so as to allow the correct positioning of the TDM frame into the multiframe memory. In the previously mentioned APON system however, the TDM frame is limited in the sense that only 3 bits can be included for this identifier as is standardized by the ITU-T specification 363.1. This makes it nearly impossible to use this prior art solution since with 3 bits only 8 different memory addresses can be accessed.

[0003] The additional delay is thereby also not acceptable for envisaged applications such as leased line services where delay requirements of 625 to 650 microseconds are common.

[0004] It is therefore an object of the present invention to provide a method, and a frame receiver device of the above known type, which is less complex, which provides less delay than the prior art method and which allows to transmit less identification bits than are strictly necessary for identifying each frame position into the multiframe.

[0005] According to the invention, this object is achieved due to the fact that said method further includes the steps as is described in the characteristic part of claim 1, and that the frame receiver device further has the features as described in the characteristic part of claim 8.

[0006] In this way, the use of synchronisation between the transmitter and the receiver, in conjunction with specific algorithms for determining the identifier value, and for writing and reading the frames in and out of the temporary buffer thereby allow, with a minimum on storage area and delay, to have these incoming frames to be placed in the correct multiframe memory location. The temporary buffer thereby has substantially less memory locations than the number of frames constituting the multiframe structure. As will become clear in a further paragraph, the specific writing and reading addressing algorithms for this temporary buffer are such that the frames which are subsequently read out from this temporary buffer for further storing one after the other into the multiframe buffer, are placed in the latter in their originally transmitted sequence.

[0007] A further characteristic feature of the present invention is described in claims 2 and 9.

[0008] This represents a lower boundary condition both for the number of memory locations in the temporary buffer, as well as for the amount of values this identifier can have.

[0009] Still a further characteristic feature of the present invention is described in claims 3 and 10.

[0010] By taking into account the number of memory locations within this temporary buffer, the value of the inserted identifier, the maximum number of different identifier values, and a recorded amount of transmitter clock pulses, a simple write addressing scheme is possible, yet allowing for a small temporary buffer. It is to be remarked that throughout this document the term “cell delay”, which is a commonly standardized feature in telecommunications applications, means the maximum delay a frame can experience between transmitter and receiver.

[0011] Claims 4 and 11 represent an example of such an algorithm.

[0012] Yet another characteristic feature of the present invention is described in claims 5 and 12.

[0013] Thereby the read addressing scheme for transfer of frames out of the temporary buffer into positions into the multiframe buffer, is calculated from the recorded amount of transmitter clock pulses. This allows for a very simple algorithm, for example represented by the formula claimed in claims 6 and 13.

[0014] Claims 7 presents a simple algorithm for calculating the identifier value, whereas claim 14 provide a simple and cheap solution for obtaining the recorded amount of transmitter clock pulses.

[0015] The present invention as well relates to a transmitter which is adapted to transmit subsequent frames of a multiframe to a receiver and to insert in each of them an identifier with an amount of distinct values which is less than the amount of frames within the multiframe structure, and which is further adapted to transmit the frames at the rate of a particular clock , which is synchronous with a receiver clock. The present invention also relates to receiver device including the previously described frame receiver device, as well as to a communications network including such a transmitter and receiver device.

[0016] The above and other objects and features of the invention will become more apparent and the invention itself will be best understood by referring to the following description of an embodiment taken in conjunction with the accompanying drawings wherein:

[0017]FIG. 1 schematically depicts a communications network in accordance to the present invention, including a receiver RX with a frame receiver device FR according to the present invention and a transmitter TX according to the present invention, and

[0018]FIG. 2 illustrates the method by means of an example.

[0019] The present invention is used in applications where individual frames, part of a multiframe structure, that are transmitted in a particular sequence, are to be recombined in this sequence such as to reconstruct this multiframe. It is to be remarked that throughout this document, the term “frame” indicates any data packet of a fixed length, whereby a multiframe corresponds to a longer structure being composed of a predetermined number of such packets. The invention is thus applicable to for instance 2 Mbps CAS TDM frames which form part of a multiframe structure with 16 frames, but also to ITU-T X.50 frames which form part of a multiframe structure with 20 or 80 frames, robbed bit signalling channels which form part of a multi-frame structure with 6 or 12 frames and other examples which are known to a person skilled in the art.

[0020] During their transmission towards the receiver, the different frames may suffer from cell delay and cell delay variation problems. As is previously mentioned in this document, cell delay is to be understood as the time, expressed in number of transmitter clock pulses, between the moment a frame leaves the transmitter and the moment this same frame arrives in the receiver. Cell delay variation is to be understood as the variation that can occur between different transmission times such frame can experience. The situation where packets, cells or frames are subject to different transmission times between transmitter and receiver happens for instance in all ATM-based networks such as APON networks, HFC which stands for Hybrid Fiber Coax networks, ADSL which stands for asymetric digital subscriber line networks, frame relay networks, IP networks . . . Cell delay variation can result in a change in the order of receipt of the cells, with respect to their transmission sequence order. To cope with such problems, in prior art solutions each frame is attributed an identifier corresponding to the sequence number of the frame in the multiframe structure. At the receiver, the individual frames are then temporarily buffered until they are all received, after which step the reconstruction of the multiframe can take place based on this sequence number or identifier. This however consumes time, and buffer memory, especially when a lot of frames form part of such a multiframe.

[0021] Another difficulty occurs when the number of bits reserved for this identifier is smaller than the amount of frames in the multiframe structure, making this straightforward solution even impossible.

[0022] The subject method, as well as the subject receiver and transmitter, are however able to cope with these problems. The transmitter is schematically shown in FIG. 1 as TX, the receiver as RX. The receiver RX includes a frame receiver device FR, having an input terminal IN on which the frames transmitted by TX are received and stored in a temporary buffer memory CM being part of a temporary buffer arrangement C. In one embodiment this temporary buffer memory consists of a circular buffer memory, but other memories are as well possible. The number of memory locations x in this circular memory is however smaller than the amount of frames f included in the multiframe, yet larger than or equal to the maximum cell delay d expressed in number of transmitter clock pulses. In FIG. 1 the memory locations in CM all have an address ranging from 0 to x−1.

[0023] For practical reasons it may indeed be sometimes more convenient to choose a circular buffer with more memory positions than the strict miminum being the cell delay parameter d. This is for instance the case when d is no power of 2. Choosing a memory with an amount of memory positions which is slightly larger than d, but being a power of 2, is a more cost-effective solution since these memories are more commercially available and are thus cheaper.

[0024] According to a first aspect of the invention, the transmitter and the receiver are synchronised. This can occur in a lot of different ways, for instance as specified by the ITU-T specification G.983.1 chapter 8.3.5.3.4. for APON systems. This synchronisation results in synchronised clocks at the transmitter and the receiver. These clocks are respectively denoted CLKTX and CLKRX, whereby their synchronisation is symbolised by the broken line between them. At the rate of this clock the transmitter sends the individual frames, wherein an identifier c is included. This identifier has an integer value between 0 and m−1, m thus being the maximum amount of values c can have. For embodiments where c is represented as a series of bits, this number of bits b is thus such that m=2^(b). In general m is limited by the amount of bits reserved for this identifier within each frame, which is standardised in most cases.

[0025] It is important however to remark that the method is valid as long as this value m is strictly larger than d, d being the maximum cell or frame delay, expressed in number of clock periods, between transmitter and receiver, whereby subsequent frames are transmitted during subsequent clock periods. This value d is guaranteed for the communications network. The value of this delay is in general dependent on the characteristics of the transmission medium used between the transmitter and the sender.

[0026] At each clock pulse, a frame is thus sent. The value of the identifier c of the frame transmitted at each clock pulse is calculated as

c=i modulo m  (1)

[0027] with i representing a recorded amount of transmitter clock pulses, which, if the transmitter clock is reset at the start of the transmission, and if each clock pulse a frame is sent, also corresponds to the number of transmitted frames so far. This value can correspond to the sequence number of the clock pulses thus provided that the transmitter clock is reset at the start of transmission. In the embodiment depicted in FIG. 1 i is obtained as the output value of a counter CNTT counting at the rate of the clock CLKTX, increasing its value at each clock pulse. During the first transmission clock period i=0.

[0028] The value of the identifier c is calculated in an identifier calculating means CIDEN, at the rate of the clock, and receiving the subsequent values of i. The thus calculated values of c are inserted in each to be transmitted frame by means of an identifier insertion device denoted FID. The latter receives the subsequent frames from a transmit buffer TB. The flow of incoming frames, generated by other circuitry (not shown on FIG. 1) within TX is shown by the thick grey arrow arriving at TB. FID, besides inserting the identifier c at the rate of the clock clk, is also further adapted for the transmission of the thus adapted frame to the receiver RX, also at the rate of the clock. The flow of transmitted frames is also schematically represented by the thick grey arrow between transmitter and receiver. The clock pulse signal is represented as the thin dashed line between CLKTX, CNTT, CIDEN and FID.

[0029] It is to be remarked that in formula (1), the modulo operation indicates the integer and positive rest of the division of i by m.

[0030] As already mentioned, the frame receiver device FR of the receiver includes a circular buffer arrangement with a circular buffer CM having x memory positions. Within this circular buffer arrangement C the subsequent incoming frames are first received within an identifier extracting means, denoted by IEM. This device has an input terminal IN and is adapted to extract from each incoming frame its identifier c, at the rate of the clock clk, delivered by the receiver clock CLKRX. The value of this extracted identifier c is then provided to a write addressing means WM. WM calculates from c, from the value of x, from the value of i provided by a similar counter CNTR, and from the value of m, the address of the position in which the received frame or frames are going to be written. Since x and m are constant values, these can be stored within WM. During each clock period, a read addressing means RM calculates as well the address of the position from which a frame is going to be read from CM for placement into the multiframe buffer memory MF of the frame receiver device FR.

[0031] In the depicted embodiment the memory CM of the circular buffer arrangement includes a number of write addressing lines, controlled by the write addressing means, and a number or read addressing lines, controlled by the read addressing means. At each clock period the write and read addresses are thereby calculated by the respective means WM and RM, and are provided as control signals pw and pr, thereby controlling the write access and read access lines of this memory CM.

[0032] The write and read addressing algorithms will now be discussed.

[0033] This receiver has a circular buffer of x positions, with x larger or equal to d, and smaller or equal to the amount of frames f in the multiframe. Every memory position of this circular buffer can contain one frame. These positions are having an address, numbered from 0 to x−1. Two activities are going on in this circular buffer:

[0034] at the beginning of every clock period, the frame in position with the address

pr=i modulo x  (2)

[0035] whereby i represents the recorded amount of transmitter clock pulses. is read out from the circular buffer CM and written in the next free position of the multiframe buffer MF, the position from which this frame is read out is erased;

[0036] when a frame containing an identifier with value c is received during this clock period , this cell is written at the position with the address pw according to the following write addressing algorithm:

pw=(((c+x−i−1)modulo m)+i+1)modulo d  (3)

[0037] with pw representing the write position address within said circular buffer into which this incoming frame is to written,

[0038] m represents the amount of distinct values c can have

[0039] c represents the value of the identifier included in the incoming frame

[0040] i represents the recorded number of transmitter clock pulses.

[0041] This value can be derived from a counter CNTR counting at the rate of the receiver clock CLKRX. Since the receiver clock is synchronised with the transmitter clock, the “i” values thus obtained are the same in transmitter and receiver.

[0042] During the first clock period, this value is 0, during the second clock period this value is 1, etc.

[0043] The mechanism of which frames are written and read out of the circular buffer can be followed in FIG. 2. Therein the different clock pulses at the transmitter TX with the corresponding values of i are schematically shown in the upper line, as well as the frames which are transmitted in each clock period by TX. The next line shows the same clock pulses, again with the corresponding value of the counter i, and also indicates which cells are received within which clock period. In the following line the frame identifier c is shown for the cells that are received, whereby the frame identifier was earlier calculated in the transmitter in accordance to formula (1).

[0044] Then the circular buffer memory CM is shown for the case where x=d=6, and for m=8. CM thus has 6 memory positions, numbered from 0 to 5. It is assumed that initially this buffer is empty. During each clock cycle, the identifier of the frames that are written in CM is denoted in italic and is surrounded by a rectangle, whereas the cells that are read out from this buffer are denoted bold. In case a frame is read from a particular position, whereby this same position is next filled by another frame, this is indicated by the “/” in between both identifiers.

[0045] At the beginning of the first clock pulse, where the value of i is thus 0, the contents of buffer position having address pr=0 is read out from CM for transfer to MF. Since at that moment nothing was present in the buffer, nothing is read out, as is represented by the dash. During this same first clock cycle, the write address pw is calculated in accordance to formula (3), resulting in that the cell that has arrived, having identifier 0, is also placed in memory position 0. This is shown by the itialic “o” , surrounded by the rectangle, that is placed in buffer position with address 0. During the next clock cycle, the contents of buffer position with address 1 is first read out, represented by the “−” since this position was empty; whereas the received cell having identifier “1” is also written in this same position, as represented by the italic “1” surrounded by the rectangle. The cell with identifier “0” in position 0 is kept there. This mechanism can be further followed, and it will be apparent that although frames that are received in a different sequence as the one with which they were previously transmitted, for instance cells with identifier 3 and 4, are still read out for transmission to the multiframe memory in their original sequence, i.e. frame with identifier 3 is read out before frame with identifier 4. Thus the original sequence of transmission is preserved, i.e. the order with which cells are read out to the multiframe memory is 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7 whereby the second sequence with identifiers 0 to 7 corresponds to the 8 to 16 th frames transmitted. It can also be observed that even if different frames are received during one clock cycle, that these are still written at separate locations such as to enable them to be read out in their original sequence, for instance cells with identifier 2 and 4, received during clock pulse with i=15.

[0046] It further is to be remarked that the modulo operation in this invention always calculates the positive rest. Thus (−10)modulo 8=6, (−6) modulo 8=2, 6 modulo 8=6.

[0047] While the principles of the invention have been described above in connection with specific apparatus, it is to be clearly understood that this description is made only by way of example and not as a limitation on the scope of the invention, as defined in the appended claims. 

1. Method for assembling frames of a multiframe, said method including a step of calculating, for each of said frames, the value of an identifier (c) for inclusion into said each of said frames before transmitting them by a transmitter (TX) at the rate of a transmitter clock (CLKTX), said method including a step of temporarily storing said frames into a temporary buffer (CM) of a receiver (RX) before reading them out from said temporary buffer (CM) for storage to a multiframe buffer memory (MF) of said receiver (RX), characterised in that said method includes a step of synchronising a receiver clock (CLKRX) with said transmitter clock (CLKTX), said identifier is calculated in accordance to a predetermined algorithm such that the amount of distinct values (m) said identifier can have is less than the amount of frames (f) within said multiframe, said method includes a further step of extracting said identifier from said frames, within said receiver, for determining a position in said temporary buffer (CM) for storing said incoming frames, whereby the amount of memory locations (x) of said temporary buffer is smaller than said amount of frames (f) within said multiframe, whereby the frames are read from positions in said temporary buffer for storage into subsequent locations into said multiframe buffer memory, irrespective of said identifier value (c).
 2. Method according to claim 1 characterised in that said amount of memory locations (x) of said temporary buffer (CM) is not smaller than the maximum cell delay (d) between said transmitter (TX) and said receiver (RX) and said amount of distinct values (m) said identifier (c) can have is larger than said maximum cell delay (d).
 3. Method according to claim 2 characterised in that a write position address (pw) of a memory location within said temporary buffer (CM) into which an incoming frame is to be written, is calculated from said amount of memory locations (x) within said temporary buffer (C), from the value of said identifier (c) included in said incoming frame, from said amount of distinct values (m) said identifier can have, and from a recorded amount of transmitter clock pulses (i).
 4. Method according to claim 3 characterised in that said write position address (pw) is calculated in accordance to the following formula: pw(((c+x−i−1)modulo m)+i+1)modulo d  (3) with pw representing said write position address within said temporary buffer into which an incoming frame is to be written, m represents said amount of distinct values c represents the value of said identifier included in said incoming frame i represents said recorded amount of transmitter clock pulses (i), x represents said amount of memory locations within said temporary buffer (C).
 5. Method according to claim 2 characterised in that a read position (pr) address of a memory location within said temporary buffer (CM) from which an already stored frame is going to be read for subsequent transmission into said multiframe buffer memory (MF) is calculated from said amount (x) of memory locations within said temporary buffer (CM) and said recorded amount of transmitter clock pulses (i).
 6. Method according to claim 5 characterised in that said read position address (pr) is calculated in accordance to the following formula: pr=i modulo x whereby pr represents said read position address whereby i represents said recorded amount of transmitter clock pulses, whereby x represents said amount of memory locations within said temporary buffer.
 7. Method according to claim 1 characterised in that said value of said identifier (c) is calculated in accordance to the following formula: c=i modulo m whereby i represents said recorded amount of transmitter clock pulses, whereby m represents said amount of distinct values said identifier canhave
 8. Frame receiver device (FR) including an input terminal (IN) adapted to receive incoming frames of a multiframe, previously transmitted by a transmitter(TX), a temporary buffer arrangement (C) coupled to said input terminal, and including a temporary buffer (CM) adapted to temporarily store said incoming frames, a multiframe buffer memory (MF) coupled to said temporary buffer arrangement (C) and adapted to store said incoming frames received from said temporary buffer arrangement (C), characterised in that said frame receiver device (FR) includes a receiver clock (CLKRX) which is synchronised with said transmitter clock (CLKTX), said temporary buffer (CM) has an amount of memory locations (x) which is smaller than the amount of frames (f) within said multiframe, said temporary buffer arrangement (C) includes identifier extraction means (IEM) adapted to extract an identifier (c) from each incoming frame, whereby the amount of distinct values (m) said identifier can have is less than the amount of frames (f) within said multiframe, said temporary buffer arrangement (C) includes write addressing means (WM) adapted to calculate from the value of said identifier from a respective incoming frame, a write position address within said temporary buffer for storing said respective incoming frame, said temporary buffer arrangement (C) includes read addressing means (RM) adapted to calculate a read position address of a memory location within said temporary buffer (CM) from which an already stored frame is going to be read for subsequent storage into said multiframe buffer (MF), irrespective of the value of the identifier of said already stored frame.
 9. Frame receiver device according to claim 8 characterised in that said amount of memory locations (x) of said temporary buffer is not smaller than the maximum cell delay between said transmitter and said frame receiver device and said amount of distinct values (m) said identifier can have is larger than said maximum cell delay.
 10. Frame receiver according to claim 9 characterised in that said write addressing means (WM) is further adapted to calculate said write position address (pw) from said amount (x) of memory positions within said temporary buffer, from said amount of distinct values (m) said identifier can have, and from a recorded amount of transmitter clock pulses (i).
 11. Frame receiver device (FR) according to claim 10 characterised in that said write addressing means (WM) is further adapted to calculate said write position address (pw) in accordance to the following formula: pw=(((c+x−i−1)modulo m)+i+1)modulo d with pw representing said write position address within said temporary buffer, m represents said amount of distinct values c represents the value of said identifier of said incoming frame i represents said recorded amount of transmitter clock pulses x represents the amount of memory location within said temporary buffer
 12. Frame receiver device (FR) according to claim 9 characterised in that said read addressing means (RM) is further adapted to calculate said read position address, from said amount of memory locations within said temporary buffer and from said recorded amount of transmitter clock pulses (i).
 13. Frame receiver device according to claim 12 characterised in that said read addressing means (RM) is further adapted to calculate said read position address in accordance to the following formula: pr=i modulo x whereby pr represents said read position address whereby i represents said recorded amount of transmitter clock pulses whereby x represents said amount of memory locations within said temporary buffer.
 14. Frame receiver device (FR) according to claims 10 or 12 characterised in that said frame receiver device includes a receiver counter (CNTR) adapted to calculate said recorded amount of transmitter clock pulses (i) as an amount of elapsed clock pulses of said receiver clock (CLKRX) included in said frame receiver device (FR).
 15. Transmitter (TX) of a communications network, adapted to transmit subsequent frames of a multiframe to a receiver device (RX), said transmitter (TX) being adapted to calculate and to insert, within each of said subsequent frames, an identifier (c), before transmitting said frames at the rate of a transmitter clock (CLKTX) characterised in that the amount of distinct values (m) of said identifier is less than the amount of frames within said multiframe, whereby said transmitter clock (CLKTX) is synchronous with a receiver clock (CLKRX) of said receiver device (RX).
 16. Receiver (RX) of a communications network, adapted to receive frames of a multiframe, previously transmitted by a transmitter of said communications network characterised in that said receiver (RX) includes a frame receiver device (FR) which is adapted in accordance to any of the claims 8 to 14 .
 17. Communications network including a transmitter (TX) adapted to transmit frames of a multiframe, and a receiver (RX) adapted to receive incoming frames for subsequent assembly into said multiframe, characterised in that said transmitter (TX) is further adapted in accordance to claim 15 and said receiver is further adapted in accordance to claim 16 . 